Many power electronic systems are currently limited by the low dielectric constant and/or low upper limit of operating temperature of commercially available capacitors and dielectric materials. Today's capacitors are typically limited in temperature capability to a maximum of 125° C., whereas many current commercial and military application temperatures range from −55 to 200° C. and even to 300° C. in some instances. Moreover, the application temperature refers to ambient service conditions and does not take into account internal heating of the part and surrounding components on the same circuit board. Accordingly, actual part temperature can readily exceed the environmental specification.
Today's capacitors that meet the higher and wider temperature range have to be fabricated from very low permittivity dielectrics and thus have relatively large dimensions. Specific uses include DC and AC power filtering, energy storage, high repetition rate (pulse power) devices, high energy back-up or hold-up power devices and small signal capacitor applications for controls. Although some of these applications may be accommodated with engineering “work-arounds” and performance trade-offs, future systems—particularly oil well logging instrumentation, avionic controls, advanced weapon electronics, power-conditioning electronics and under-the-hood automotive components—will require better material properties to meet more demanding performance goals.
An increasing problem in certain systems today is exceeding the service temperature of electronic components because of the heat generated in current highly miniaturized and densely packed electronic circuits. For example, temperature limitations of capacitors have been identified as the weak link in power electronic system reliability as manifest in motor drives, inverters/converters for switched reluctance starter/generator systems, DC to AC converters, DC to DC converters, and pulse forming networks.
Instead of utilizing remote processing and control, many industrial and commercial applications are now being designed to take advantage of the weight, power and space savings that can be achieved with local dedicated electronic boxes. For example in transport applications, such as automobile motors and aircraft engines, sensing and control instrumentation is being placed closer to the power plant itself where the service temperature limits exceed those of available electronic components. Similarly, the large oil and gas exploration companies are placing bore-hole instrumentation directly adjacent to the hot zone at the drilling face.
While barium titanate has been used in high performance capacitor applications and has a wide operating temperature, ultimate performance is limited by its inherently low transition temperatures (−90, 0, 130° C.) and its semiconducting properties at elevated temperature.
Many other high temperature electroceramic materials are known which have isolated high dielectric performance over a limited temperature range but very low dielectric constant at temperatures on either side of this band. An example is lead titanate which is an excellent dielectric in the very close vicinity of its 490° C. phase transition temperature. Attempts have been made to broaden the dielectric response, even though this lowers the temperature at which the dielectric constant peaks, by modification with calcium, but its dielectric properties then drop into the low hundreds.
Alternatively many electroceramic materials are known to have constant but low permittivities over a wide range of temperatures. Thus dielectrics based on medium-K titanates such as strontium titanate (SrTiO3 or ST), calcium titanate (CaTiO3 or CT), magnesium titanate (MgTiO3 or MT), glass-ceramics and porcelains have been formulated to have near zero variation in permittivity with temperature and low loss over a wide temperature range. These dielectrics typically have dielectric constants<200 and so capacitors made from them are relatively large.
A need exists for a moderately-high permittivity dielectric that has low variation in dielectric constant with temperature, low loss and high resistivity over a broad temperature range.